1. Field of the Invention
This invention relates to a process for purifying 4,4'-benzophenonedicarboxylic acid, useful in preparing thermoplastic polyesters.
2. Brief Description of the Prior Art
It is known in the art that in order to optimize the properties of thermoplastic polyesters to be used for making shaped articles, it is usually necessary to employ substantially pure starting materials to avoid crosslinking during the molding process. This is especially true for aromatic dicarboxylic acid-based polyesters which are to be shaped into articles for use in applications requiring high impact resistance, e.g. safety glass, windows, housing for chemical machinery and the like.
Aromatic dicarboxylic acids used in preparing polyesters, are normally obtained by oxidation of the precursor dialkylaromatic hydrocarbon. So obtained, they are generally contaminated with genetic impurities, particularly, incompletely oxidized aromatic monocarboxylic acids. In the case of 4,4'-benzophenonedicarboxylic acid, a common genetic impurity is 4-alkyl-4'-benzophenone monocarboxylic acid. Monocarboxylic acid genetic impurities contained in the final dicarboxylic acid product must be removed before use in polymerization to insure desirable properties of the final polymer. However, separation of the pure diacid from its genetic monocarboxylic acid impurities often is a tedious and difficult task.
Prior art processes for the purification of 4,4'-benzophenonedicarboxylic acid are known and are described in the following patents: U.S. Pat. No. 3,197,499 (McCracken, et al., 1965) which involves preparation of the dimethyl ester; U.S. Pat. No. 3,510,513 (McCracken, et al., 1970) which uses hydrogen and a hydrogenation catalyst at high temperature and high pressure; U.S. Pat. No. 3,370,088 (Lese, et al., 1968) which involves high pressure extraction of impurities with methanol or acetone; U.S. Pat. No. 3,577,457 (Schultz, 1971) which involves heating with methanol or ethylene glycol to remove impurities; and U.S. Pat. No. 3,448,146 (Lese, et al., 1969) which involves forming a carbonyl adduct of the impurities.
However, the above methods possess the disadvantages of requiring either repeated applications for obtaining high purity, or expensive high pressure equipment, high temperatures, expensive organic solvents, or conversion of the diacid to the diester necessitating a subsequent saponification step, all of which render the methods expensive, hazardous or time consuming on an industrial scale.
A process for purifying 2,6-naphthalenedicarboxylic acid is disclosed in U.S.S.R. Patent No. 333,164 (1972), wherein crude 2,6-naphthalene dicarboxylic acid is purified from isomeric naphthalene dicarboxylic acids by dissolving alkali salts of these acids in an aqueous alcoholic medium containing up to 80 weight percent ethanol or ethylene glycol, followed by saturation of the resulting solution with ammonia gas and separation of the desired product from the resulting precipitate. However, the process requires the use of large amounts of expensive organic solvents.
Related methods are known for the purification of low molecular weight aromatic dicarboxylic acids, such as terephthalic acid, but because of large differences in melting points and solubilities between low and high molecular weight aromatic dicarboxylic acids, it is generally not predictable that these related methods will also be applicable to high molecular weight acids.
Representative purification methods for terephthalic acid containing impurities such as para-toluic acid and isophthalic acid, obtained by oxidation of commercial p-xylene by means of air or nitric acid are described in the following patents: U.S. Pat. No. 3,215,734 (Katzschmann, 1965) in which a crude oxidation mixture is suspended in a water-dimethylformamide mixture, ammonia gas is bubbled through forming the soluble ammonium salt of terephthalic acid at about 90.degree. to 100.degree. C., and water is then distilled off until the ammonium salt of terephthalic acid precipitates; U.S. Pat. No. 3,849,489 (Rudzki, 1971) in which a two-step process is utilized forming an ammonium salt of crude terephthalic acid and then volatilizing off impurities by heating at a temperature of 280.degree. to 290.degree. C., and then decomposing the ammonium salt to terephthalic acid, wherein the process is then repeated but the separation step is accomplished by bubbling ammonia into the solution to separate the ammonium salt of terephthalic acid, which is recovered and converted into terephthalic acid; U.S. Pat. No. 2,862,963 (Fuchs, et al., 1958) in which a crude mixture of acids is suspended in a liquid medium consisting of one or more alcohols containing up to six carbon atoms, saturating the mixture with gaseous ammonia at elevated temperatures thereby forming soluble ammonium salts of monocarboxylic acids and the insoluble ammonium salt of terephthalic acid.
Russian Patent No. 252,324 (1970) describes the separation of terephthalic acid from the products of the thermocatalytic regrouping of the potassium salts of benzenecarboxylic acids by dissolving the potassium salts in aqueous solution, adding ammonia to form the diammonium salt of terephthalic acid and preferentially precipitating it with the addition of acetone. However, the method is not described as being applicable to a mixture of the free acids containing a substantial amount of aromatic monocarboxylic acid impurity.
While the above related methods are useful for the purification of terephthalic acid, no suggestion is made that the methods are useful for the purification of 4-4'-benzophenonedicarboxylic acid.